1. Field of the Invention
The invention relates to a process and a device for gravure printing with an erasable and reusable gravure form, starting from a blank gravure form with a basic screen that is designed for at least the maximum quantity of ink to be transferred.
2. Description of the Prior Art
Gravure printing is a printing process in which the printing elements are more deeply inlaid than the printing form surface. After the printing form has been completely inked, its surface is cleared of printing ink, so that ink only remains in the depressions. Gravure printing forms include, for example, copper-plated steel cylinders, hollow cylinders slipped onto clamping cores and, in some cases, copper sheets clamped onto cylinders.
The manner of inking and clearing the ink from the form surface by blades permits no pure surface printing. The entire drawing must be resolved into lines, points or screen elements. Because of their different depths and sizes, individual printing elements hold more or less ink. The printed image therefore has different ink strengths at different points.
Various methods are used today to produce gravure forms. For example, among the variabledepth processes, there is the etching method, which consists of the slow diffusion of concentrated iron chloride solutions through a pigment gelatin layer. The pigment copy on the copper printing form consists of a solidified gelatin relief, which corresponds to the tonal gradations of the diapositive. The engraving process is distinguished by the line-by-line scanning of image and text with photo cells and the simultaneous engraving of the printing form with engraving heads. Special mention should be made of the production off depressions in the copper layer of the printing form by means of a high-energy electron beam, which is directed onto the blank form in a vacuum and removes material from the form in accordance with the image. Printing forms engraved in this manner can be provided with screens of variable depth and area.
It is also possible to create depressions by means of a high-energy laser beam. In this case, measures must be taken to ensure the input of the laser energy into the substrate, because without special pretreatment, copper, in particular, reflects laser beams to a great extent.
Furthermore, German reference DE OS 27 48 062 discloses a process for producing an engraved printing form as follows: First, a blank gravure form is prepared, in that its smooth surface is provided evenly with depressions of the same depth and size. Then, the engraved surface is covered with a light-sensitive mass so that all depressions are filled. After this, the blank form is exposed photographically to the desired image, so that the exposed areas polymerize. The unexposed portions can be washed out and image differentiation is thus achieved.
In general, in all gravure printing processes, the image locations of the printing form lie deeper than non-image locations. Particularly in blade gravure printing, the screen network is formed by stems of equal height, which border the image locations and form a resting surface for the blades. A separate set of printing form cylinders is needed for each print job (one printing form cylinder per ink color, with a corresponding number of print sides). These cylinders have the particular circumference required for the printing format in question. When a gravure printing machine or a rotary printing machine is readied for use, the appropriate printing form cylinders must be exchanged. These cylinders, which have a width of 200 cm, for example, currently weigh approximately 800 kg. Because the processes described above must be carried out outside of the printing machine, high mechanical expense is involved. In addition, each of these production methods includes steps, such as galvanization, coating, exposure and development, which make it impossible to reuse the same printing form without extensive--and, in part, chemical--treatment. Moreover, after etching or engraving in accordance with the image takes place, i.e., after material is removed, chrome-plating is usually performed to increase the serviceable life.
As a rule, if a printing form is to be stored for repeat jobs, space must be prepared for the entire cylinder. Printing form production is complicated and thus expensive, particularly when galvanic steps are required. Moreover, the toxic slurries created during production are an ecological problem.
In contrast, German reference DE 38 37 941 C2 discloses a process for producing a gravure form that allows imaging to be carried out directly in the printing machine. This process also permits the image on the gravure form to be erased in the printing machine and the form to be prepared there for a new image. Moreover, a blank gravure form is produced with a basic screen designed for at least the maximum quantity of ink to be transferred. In the printing machine, a thermoplastic substance is introduced into the depressions, from a nozzle of the pixel transfer unit or by means of image-correlated impressing, in a quantity inversely proportional to the image data for the purpose of reducing the scoop volume of the depressions. In other words, this method, in contrast to the others, calls for material to be applied to a blank gravure form in accordance with the image. After the printing job, it is possible to liquify the thermoplastic substance using a heat source in the printing machine, and then to remove the liquified substance from the printing form cylinder by a wiping and/or blowing or suctioning device.
However, applying material in accordance with the image poses problems with respect to the accuracy of the image position. Without further measures, it is not possible to completely introduce material stored on the stems into the depressions. Nonetheless, complete introduction is necessary if the transferred material as a whole is to help reduce the scoop volume of the depressions as desired.
In an earlier German Patent Application P 195 03 951, it has already been proposed to evenly fill the depressions in the basic screen of the blank gravure form with a liquefiable substance by accordance with the image by means of a pixel transfer device. Next, the screened gravure form is inked in accordance with the image by means of an inking system. Then, printing takes place using the gravure process, after which the blank gravure form can be regenerated and the depressions can again be filled evenly.
The liquefiable substance for filling the basic screen can, for example, be a thermoplastic material. The substances that can be used include thermoplastics (plastomers), e.g., polyolefins, vinyl polymers, polyamides, polyesters, polyacetates, polycarbonattes and, in some cases, polyurethanes and ionomers, as well as hot melt (wax), lacquer or an interconnectable polymer melt or solution.
A laser, preferably an NdYAG or an NDYLF laser, is used to burn the filled gravure form free in accordance with the image.
Inking with aqueous printing ink is carried out by means of a chamber blade, and a print stock is printed, preferably by indirect gravure printing.
After the required printing sequence, the gravure form is cleaned of ink residues by a regeneration device, preferably an ultrasonic cleaning unit, and the liquefiable substance is removed from the depressions of the basic screen. This allows the cycle of filling, ablation in accordance with the image, inking, printing and regeneration to begin anew.
Image erasure (regeneration) is also possible by completely cleaning the basic form with a pressurized water jet from a high-pressure cleaner. To this end, an arrangement such as that disclosed in European reference EP 9 310 798 is used. Such an arrangement comprises a double-walled chamber that is open toward the gravure form, but closed relative to the surrounding environment by seals running across the form. The inner cell contains nozzles, through which water is sprayed at high pressure onto the surface of the gravure form. There is suctioning out of the outer chamber area, which is surrounded by a mantle, so that fluid is removed, particularly from the area already cleaned, and the gravure form is clean and dry after treatment.
The high-pressure cleaner can work on at least two different levels. One level essentially uses a low liquid pressure and/or temperature to remove the remaining ink, while the other levels use a higher liquid pressure and/or temperature to partially or completely remove the filling material.
The above statements all relate to an implementation of the described measures inside of a gravure printing machine. It is understood, however, that the described measures can be implemented outside of a printing machine as well.
In such rotary printing processes, as a rule, an ink that contains a solvent (even water-based printing inks contain certain amounts of solvent) is transferred onto the printing stock. The solvent must be then extracted from the printed stock and from the ink layer on or partly` penetrating the stock. This is done in a drying section of the printing machine. The large amount of space required for a drier and the high expenditure of energy for drying are disadvantageous. In addition, the emitted solvents pollute the environment. Furthermore, when the ink film of a water-based printing ink is dried, amines used for saponification and ammonia also escape into the ambient air. These released compounds are not only malodorous, but also hazardous to health.
Furthermore, until now it has always been necessary to use at least three different process media to regenerate the basic form, especially when high-pressure cleaners are used. These media are the erasure fluid (water), the printing ink (usually a solvent-containing ink) and the filler (liquefiable substance). A certain risk of the process media becoming- mixed during production therefore exists.
A further development of printing inks focuses on inks with UV solidifiable binding agent systems, which make use of the generally known principle of UV solidification.
For example, German reference DE 43 07 766 C1 describes a process for producing a UV solidified flexographic printing ink and its use in flexographic printing machines.
The UV printing inks contain inking substances and additives, along with photo initiators, which, when subjected to UV radiation, trigger polymerization of the binding agent components also contained in the ink. As described in DE 43 07 776 C1, UV systems of this type can polymerize by a radical or by a cationic mechanism. UV inks contain no solvents or water. Technical, health-related and environmental disadvantages and problem, such as are caused by solvents or water in printing inks, do not arise in connection with UV printing inks.